1. Field of the Invention
The present invention related to a molding die for resin sealing. More particularly, the present invention relates to a two-gate molding die for resin sealing in which gates are provided in an upper mold half and a lower mold half on the upper and lower sides of a leadframe, and a method of manufacturing a semiconductor device using the same.
2. Description of the Related Art
Conventional molding dies for resin sealing are of the single-gate type such as a lower-gate type in which a gate is provided below a leadframe, or an upper-gate type in which a gate is provided above a leadframe. However, single-gate type molding dies proved to have a disadvantage of non-filling of resin as the size of a chip and a package become larger and larger. Accordingly, instead of the single-gate molding die, two-gate molding dies have come to be widely adopted.
Molds having two gates are described in some literatures. Patent Publication No. JP-A-63-228631 discloses a molding die having two gates wherein the opening areas at the upper and lower gates are made substantially the same. Utility Model Publication No. JU-A-64-47040 discloses a molding die having two gates wherein an air vent is provided at one edge of a cavity. Patent Publication No. JP-A-2-9142 discloses a leadframe having a cut-away portion for connecting upper and lower gates with each other. Utility Model Publication No. JP-A-2-186647 also describes a molding die having upper and lower gates.
A conventional molding die which has two gates will be described with reference to FIG. 1, which shows a molding die together with a semiconductor chip sealed by resin. The molding die 200 for resin sealing is comprised of an upper mold half (or cope) 202 and a lower mold half (or drag) 201. The lower mold half 201 has a lower runner 204 extending horizontally for defining a lower runner space, a lower cavity 208 for molding, and a lower gate 206 provided between the lower runner space and the lower cavity 208 for separating them, with a resin passage or gate space being left therebetween. A pot 203 is formed at the front edge of the lower runner 204, and a lower runner rising slope 213 is formed between the lower runner 204 and the lower gate 206, the rising slope 213 rising in the direction of resin flow.
The upper mold half 202 has an upper resin well 215 for introducing resin, an upper cavity 209 for molding, and an upper gate 207 provided between the upper resin well 215 and the upper cavity 209 for separating them, with a resin passage or upper gate space being left therebetween. The upper resin well 215 is formed as a recess defined by an upper runner 205, an upper runner falling slope formed at the front edge of the upper runner 205 and an upper runner rising slope 212 formed at the rear edge of the upper runner 205.
The molding die 200 having the lower mold half 201 and the upper mold half 202 is heated to a desired temperature before receiving resin. The sealing resin 211 is held, in a molten state, due to the high temperature of the molding die, in the pot 203 of the lower mold half 201. After a leadframe 301 on which the semiconductor element 309 has been mounted and wire-bonded is disposed on the lower mold half 201, the upper mold half 202 is lowered so as to hold the leadframe 301 between the same and the lower mold half 201. Thereafter, a plunger (not illustrated) provided below the resin 211 is raised to force the molten resin 211 to flow in the direction designated by the arrows 1A, 2A, . . . , 6A.
In detail, as a result of the upward movement of the unillustrated plunger provided below the resin, the resin 211 in the pot 203 flows along the lower runner 204 formed in the lower mold half 201 in the direction indicated by arrow 1A. The molten resin 211 then flows in the direction indicated by arrow 2A, at which the resin 211 is divided into an upper flow toward the upper gate 207 and a lower flow toward the lower gate 206. The upper flow of the resin 211 is allowed to pass through the resin passage aperture 308 formed in the leadframe 301.
The upper resin well 215 defined by the upper runner falling slope 214, the upper runner 205 and the upper runner rising slope 212, is generally designed such that the distance from the upper gate 207 to the front edge of the upper runner falling slope 214 is smaller than the distance from the lower gate 206 to the front edge of the lower runner rising slope 213, as illustrated in FIG. 1.
The resin flowing in the direction indicated by arrow 2A passes through a lower gate space formed between the lower gate 206 and the surface of the leadframe 301 and an upper gate space formed between the upper gate 207 and the surface of the leadframe 301, respectively, to enter the lower cavity 208 and the upper cavity 209, as indicated by arrows 3A and 4A. The resin flowing into the cavities 208 and 209 further flows in the directions indicated by arrows 5A and 6A toward air vents 210 which are formed to vent the air existing in the upper and lower cavities 208 and 209 before sealing to the outside of the mold. As a result, the cavities 208 and 209 are filled with resin for molding the semiconductor element.
The semiconductor element which has been subjected to the resin sealing process is taken out of the molding die after the completion of the molding process. At this time, the resin hardened in the upper resin well 215 is attached to the semiconductor element as a burr or flash. The burr should be removed in a subsequent burr removal process called degating. However, in some cases the burr remaining on the semiconductor element, is often fed to a subsequent tie-bar cutting process. In such a case, the burr falls off and is bitten by the tie-bar cutting die in the tie-bar cutting process, which causes damage to the tie-bar cutting die to stop the process.
Furthermore, in the conventional two-gate molding die, little attention has been paid to the height of the gate space, which is the distance from the surface of the leadframe to the gate, in the conventional molding die. Hence, it is difficult to obtain an equal flow rate of resin spreading into the upper and lower cavities, thereby causing a difference in charge rate of resin. The difference in charge rate between the cavities causes the resin to shift or deviate the island or the bonding wires, whereby the island or the bonding wires may be exposed at the surface of the resultant package.